Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, combustion equipment 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18 and an exhaust nozzle 19.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low-pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbine 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13, and the fan 12 by suitable interconnecting shafts 26, 27, 28.
From the above it will be appreciated that control of gas flows through a gas turbine engine is important in terms of achieving operational efficiency.
In such circumstances in order to avoid surging it is known to provide bleed valves which essentially allow when required. FIG. 2 provides a schematic side view of part of a prior gas turbine engine in which a bypass flow 30 is presented about a fairing 31 which surrounds an engine core including an intermediate pressure compressor stage 32 and a high pressure compressor stage 33. In use the gas flow pressure generated by the compressors 32, 33 acts with a combustor 34 in order to generate thrust. The casing 31 incorporates acoustic material 35 upon an outer surface with ducts 36 extending from an inner end adjacent to a respective compressor 32, 33 stage to an outer surface formed by a noise attenuation silencer 37 which is generally substantially contiguous with an outer surface 38 of the casing 31.
The inner end of the ducts 36 generally incorporates a bleed valve 39 which, as indicated, can modulate core mass flow from the compressors 32, 33 generally taken from vents 40.
It will be noted that the noise attenuation silencers are mounted with seals 41 and the acoustic material 35 generally secured appropriately through the groove or other association 42 with the diaphragm 41a. 
In the known acoustic apparatus depicted in FIG. 2, it will be understood that the valves 39 generally are located within the duct 36 and the duct 36 terminated by the silencer 37. The silencer 37 is a porous structure with small holes operating at super critical pressure ratios in order to attempt to increase the characteristic acoustic frequency of a gas flow 43 passing through the valve 39 to a frequency less critical to the human ear. Placing the flow 42, 43 acoustic frequency at a higher frequency range will also allow more effective attenuation by acoustic liners within an engine and through atmospheric acoustic attenuation. It will also be appreciated that the silencers 37 mitigate the consequences of releasing hot gases by inducing increased mixing and flow deflection in the bypass flow 30.
Prior arrangements have not been perceived as fully able to mitigate the impact of hot gas release into the flow 30 to high exit temperatures and high exit velocities through the silencer 37. It will also be understood that the silencer 37 is a single stage approach without secondary protection should the silencer 37 fail. It will also be understood that the silencer 37 is generally a single porous plate, that is to say a plate with a distribution of apertures of a desired size and spacing and so the silencer 37 generally does not maximise acoustic treatment of the flow 43 due to variations in flow rate which in turn results in the silencers 37 being prone to failure as a result of large and varying pressure loads. It will also be noted that respective ducts 36 are required for valves 39 associated with respective intermediate and high pressure stages resulting in duplication which adds considerably to weight, cost and maintainability. The ducts are generally cylindrical sleeves or tubes causing direct impingement from the gas flow source, that is to say the valve upon the silencer exit surface with apertures.